Sharmina Parvin could not have imagined that a birthday meal for her 13-year-old daughter in a Dhaka restaurant would turn into a nightmare that still hasn't ended.

On 1 July Parvin and her husband Hasnat Karim dined out at the Holey Artisan Cafe with their daughter and eight-year-old son.

What followed was a 12-hour hostage drama as Islamist militants stormed the cafe, killing 20 people.

Parvin's husband ended up in custody after security forces ended the siege and has not been released.

Answering questions submitted by the BBC in writing, Parvin recounted the night's events and appealed to the Bangladeshi authorities to release Karim, saying "he has not done anything wrong".

She described how gunmen ordered the family to put their heads down on the table. They remained in that position most of the time.

RECITING THE QURAN

During the night, not far from where the family was sitting, the militants killed nine Italians, seven Japanese, one US-Bangladeshi citizen, an Indian and two Bangladeshis.

Parvin says the gunmen did not harm them because they - along with a few other diners - were Muslims and could recite from the Quran.

Almost 11 hours later the family's ordeal ended. The gunmen released the hostages before the army launched an assault, killing all five militants.

As the hostages emerged from the compound, police took them away for debriefing.

Hasnan Karim is seen with his wife Sharmina Parvin in a family photo. Photo courtesy: Family

Parvin's husband Hasnat Karim and another man Tahmid Khan, a Canadian-Bangladeshi, were the only ones taken into custody later for further interrogation. Neither of them have returned home.

"I really do not understand why the police are still holding him. He was a hostage along with family members," Parvin told the BBC by email.

"He is a victim in all of this and has not been charged with any offence.

"My husband survived a horrifying ordeal in which several persons were murdered by armed terrorists. He has not done anything wrong."

The 47-year-old Karim has dual British-Bangladeshi citizenship. He lived in the UK for 21 years before going to the United States to study for an MBA.

Hasnat Karim in a family snap with his children. Photo courtesy: Family

He then worked as a lecturer in Dhaka's North South University, later joining his father's construction firm.

The Bangladeshi authorities have given conflicting accounts of Karim's detention.

Initially they told local media that they had kept him in custody but that he was released later on.

Later, Home Minister Asaduzzaman Khan told reporters that Karim was under investigation and still being held by the authorities.

It is not clear where Karim is being held.

When the BBC contacted various branches of the security forces, all of them said they had no information on his whereabouts.

The Dhaka Metropolitan Police Commissioner Asaduzzaman Mia told the BBC earlier this month that Karim and Tahmid were "suspects because of their behaviour on that day."

When asked whether the two were still being held by the security forces DMP commissioner said: "I don't know the exact thing… But after the incident yes we interrogated them. But (whether) they are still in police custody or not, I cannot make any specific comment. I have to know the exact situation from the investigating officer."

According to media reports, Karim was seen moving inside the cafe along with the militants during the night of the hostage taking, raising suspicion that he might be linked to them.

But Pavin calls this "complete nonsense".

"I cannot even begin to understand the basis of these allegations. Hasnat is a good person," Parvin says. On the night of the cafe siege, the gunmen used her husband as a "human shield" and took him to the terrace, she says, arguing that he simply followed their instructions.

DESPERATELY WORRIED

Human rights groups have also expressed concern.

"If the authorities do have Hasnat Karim in custody, then they must release him immediately or produce him in a court of law for any charges to be filed against him", Amnesty International said in a statement issued earlier this month.

Karim's family have also sought help from the British High Commission in Dhaka.

"Our staff in Dhaka are in contact with Bangladeshi authorities following the detention of a Bangladeshi-British citizen and have requested consular access," a spokesman for the British Foreign Office said in an e-mailed response.

Parvin says the last time the family were able to meet Karim was on 13 July. Since then, they have not received any update on his whereabouts.

"The last time I saw my husband he was very weak and exhausted both physically and mentally," Parvin says. "I am afraid for his health especially as he has a heart condition. It makes the situation all the more serious and we are all desperately worried."